The present invention is directed to a thermoplastic elastomer (TPE) composition, particularly for use in penetrable articles.
TPEs are elastic, flexible polymers that exhibit similar physical properties to elastomers, but which are recyclable and easier to process. Compositions made from TPEs are widely used in applications which have traditionally employed silicones (polysiloxanes) and other thermoset rubbers, which do not melt. Articles can be formed from TPE compositions to have similar physical properties as their elastomer counterparts while also being injection moldable, which reduces the cost of production and allows the TPE article to be recycled. TPE compositions can thus be injection molded to form articles such as seals, gaskets, septa, caps for bottles, plugs, medical devices, and other objects that might otherwise be formed from silicone.
TPEs are easier to process than elastomers because they are crosslinked by non-covalent bonds (secondary interactions). At room temperature, TPEs behave like crosslinked elastomers. However, at elevated temperatures, they behave as linear polymers. Thus, TPEs, unlike elastomers, have reversible properties as the temperature increases or decreases. The reversible crosslink allows articles formed from TPE compositions to be melted and reformed.
A TPE can be formed from block copolymers or from blocks of homopolymer having various tacticity within its chains. The elastomeric properties of thermoplastic elastomers result from the phase separation between the blocks. One of the blocks forms a continuous phase, which provides rubbery properties, while another block is glassy or crystalline. The glassy/crystalline blocks provide the crosslinks for the blocks of the continuous phase.
In the case of block copolymers, styrene is frequently used as the glassy/crystalline block that crosslinks with other polymer blocks. Block copolymers that contain styrene are known as styrenic block copolymers, or SBCs. Examples of SBCs include SBS block copolymers (styrene-butadiene-styrene), SIS block copolymers (styrene-isoprene-styrene), and SI/BS block copolymers (styrene-isoprene/butadiene-styrene). SBS, SIS, and SI/BS block copolymers can be hydrogenated to yield the hydrogenated styrenic block copolymers (HSBCs), such as SEBS (styrene-ethylenebutylene-styrene), SEPS (styrene-ethylene/propylene-3-methylbutene-styrene), SEEPS (styrene-ethylene-ethylene/propylene-styrene), and SIPS (styrene-isoprene-styrene block copolymer).
In general, TPE compositions are formed by blending TPEs, polyolefins, and additives, such as plasticizers, stabilizers, and lubricants. The polyolefin functions as the continuous phase of the TPE composition. Polypropylene is the polyolefin typically selected for the continuous phase of many TPE compositions.
It has been found that prior art TPE compositions cannot simultaneously provide desired levels of hardness, temperature stability, moldability, and tensile strength while also exhibiting satisfactory reseal properties. It would be desirable to be able to produce TPE compositions which overcome such limitations.